1. Field of the Invention
This invention relates to braking systems of electric drive vehicles, and in particular to strategies for modifying regenerative braking that might otherwise potentially affect vehicle handling when the brake pedal is depressed while the vehicle is traveling on a low Mu surface and/or while an anti-lock brake system (ABS) is actively controlling the application of friction brakes at the vehicle wheels.
2. Background Information
The powertrain of an electric drive vehicle comprises a rotary electric machine whose operation may be under the control of a powertrain control module (PCM), a name that is sometimes given to an electronic module that processes certain data to control various aspects of powertrain operation. Sometimes the electric machine operates as a motor that makes a positive torque contribution to powertrain torque. At other times the electric machine operates as a generator that makes a negative torque contribution to powertrain torque. Positive torque contribution from the electric machine may appear as traction torque delivered through a drivetrain of the vehicle to at least some of the wheels to propel the vehicle. Negative torque contribution from the electric machine imposes braking torque on the drivetrain to brake the vehicle.
When an electric machine imposes braking torque on the drivetrain to brake the vehicle, it is operating as an electric generator. Generated electricity may be used advantageously to regenerate an on-board electric power supply such as a battery. Hence, such braking is commonly referred to as regenerative braking, or sometimes simply regen for short. A vehicle that possesses regenerative braking capability typically does not rely exclusively on such braking for the vehicle service brakes. While some energy recovery is made possible by regenerative braking, it is inappropriate at certain times to invoke regenerative braking. For example, the state of charge (SOC) of a battery, or battery bank, may be such that regenerative electric current from the electric machine should not be fed, either in whole or in part, to the battery or bank. In the absence of a suitable sink for such electric current, an alternate braking means is needed.
Therefore, an electric drive vehicle typically employs some form of mechanical service brakes, such as friction brakes at individual wheels. Mechanical friction brakes may be hydraulic-, pneumatic-, or electric-operated. It is known to use an electronic brake controller, or brake control unit (BCU), to apply relative proportions of regenerative braking and friction braking when braking is called for.
The braking system may include an anti-lock capability embodied in an anti-lock brake system (ABS) that is effective to sense incipient wheel lock-up and modulate the application of the friction brake in a way that seeks to avoid wheel lock-up and resulting skidding that could possibly lead to loss of vehicle control during certain braking events. Various types of anti-lock brake systems are presently in commercial use.
When a vehicle experiences a driving situation where incipient wheel lock up is detected, the driver has typically removed his or her foot from the accelerator pedal with or without having depressed the brake pedal. In an electric drive vehicle, the action of releasing the accelerator pedal may initiate regenerative braking that simulates the compression braking that would occur in the same situation in a vehicle whose powertrain comprises an internal combustion engine. This form of simulated braking may be referred to as compression regenerative braking, or CR braking for short. Regenerative braking that results from depressing the brake pedal is referred to as service brake regen, or SBR for short.
An electric drive vehicle may have multiple higher levels of CR braking in order to increase energy recovery during a braking event and simulate the compression braking that would be imposed on the powertrain by a combustion engine when an associated transmission is in a low gear. These higher levels of CR braking, along with additional SBR, may have an unintended effect on ABS operation, and so it has heretofore been proposed to eliminate regenerative (both CR and SBR) braking in an electric drive vehicle when its ABS becomes active. When the ABS detects incipient wheel lock-up, a signal is broadcast to the PCM, which in turn immediately reduces CR braking.
A preliminary novelty search developed the following U.S. Patents as evidencing the state of the art: U.S. Pat. Nos. 4,671,577; 4,701,682; 4,799,161; 4,962,969; 5,322,352; 5,358,317; 5,378,053; 5,423,600; 5,433,512; 5,450,324; 5,476,310; 5,492,192; 5,511,859; 5,615,933; 5,632,534; 5,664,850; 5,769,509; 5,839,533; 5,839,800; and 5,857,755.
It is believed that strategies other than immediate elimination of regenerative braking may be useful in an electric drive vehicle when an incipient wheel lock-up is detected, the ABS becomes active, and/or incipient wheel slip is detected. Such strategies are the subject of the present invention.
According to a first aspect, the present invention relates to a strategy in which torque that is being applied to the drivetrain as regenerative braking torque at the time a wheel-condition-initiated event occurs is applied as friction brake torque at wheel brakes instead of as regenerative torque. A wheel-condition-initiated event may be any one of incipient wheel lock-up, ABS becoming active, and wheel slip. Implementation of the strategy can be accomplished by an exchange of appropriate information between the PCM and the BCU. A slewing technique may be used to transition the torque shifting from regenerative braking to friction braking. When the wheel-condition-initiated event ceases, a decision is made whether or not to resume regenerative braking. This decision is made on the basis of additional vehicle operating conditions not necessarily related to the wheel conditions (e.g. traction torque request from the accelerator pedal).
According to a second aspect, the present invention relates to a strategy in which the torque that is being applied to the drivetrain as regenerative braking torque at the time of occurrence of one of the above-mentioned wheel-condition-initiated events is reduced to a predetermined level, but not completely eliminated. The level to which the regenerative brake torque is reduced may be the highest amount of regenerative brake torque that will not substantially impact vehicle stability, steerability, or intended effectiveness of the ABS when the wheel-condition-initiated event occurs under any surface condition because the ABS becomes active. This too can be accomplished by an exchange of appropriate information between the PCM and the BCU. The amount of regenerative brake torque that will not substantially impact vehicle stability, steerability, or intended effectiveness of the ABS can be obtained in any one or more of various ways, such as by mathematical modeling and calculation, or by testing a vehicle under the worse case surface conditions, typically ice, to determine what the maximum tolerable amount would be without affecting vehicle dynamics.
According to a third aspect, the present invention relates to a strategy in which the coefficient of friction of the surface on which the vehicle is traveling is estimated and the regenerative braking is reduced to a value appropriate to the estimate. Such a value is one that does not substantially change the handling characteristics of the vehicle. For example, the torque to which regenerative braking is reduced may be made progressively smaller as the value of the coefficient of friction of the underlying road surface (Mu) decreases. Estimation of the value of Mu may be made on-board the vehicle by known techniques or algorithms.
According to a fourth aspect, the present invention relates to a strategy in which the coefficient of friction of the surface on which the vehicle is traveling is repeatedly estimated during activation of the ABS, and regenerative braking is adjusted according to the magnitude of wheel slip. For example, greater regenerative braking may be allowed at smaller wheel slips and lesser regenerative braking at greater wheel slips, or the regenerative braking may be adjusted in relation to the rate at which wheel slip is changing.
According to a fifth aspect, the present invention relates to the strategies described in any of the second, third, and fourth aspects wherein adjustments are made on a wheel-by-wheel basis.
Further aspects will be seen in various features of a presently preferred embodiment of the invention that will be described in detail.